1. Field of the Invention
The present invention relates to cryostats including cryogen vessels for retaining cooled equipment such as superconductive magnet coils. In particular, the present invention relates to radiation shields provided for reducing thermal radiation reaching a cryogen vessel from cryostat components which are at a higher temperature, and to venting arrangements allowing cryogen gas to escape from the cryogen vessel.
2. Description of the Prior Art
FIG. 1 shows a conventional arrangement of a cryostat including a cryogen vessel 12. A cooled superconducting magnet 10 is provided within cryogen vessel 12, itself retained within an outer vacuum chamber (OVC) 14. One or more thermal radiation shields 16 are provided in the vacuum space between the cryogen vessel 12 and the outer vacuum chamber 14. In some known arrangements, a refrigerator 17 is mounted in a refrigerator sock 15 located in a turret 18 provided for the purpose, towards the side of the cryostat. Alternatively, a refrigerator 17 may be located within access turret 19, which retains access neck (vent tube) 20 mounted at the top of the cryostat. The refrigerator 17 provides active refrigeration to cool cryogen gas within the cryogen vessel 12, in some arrangements by re-condensing it into a liquid. The refrigerator 17 may also serve to cool the radiation shield 16. As illustrated in FIG. 1, the refrigerator 17 may be a two-stage refrigerator. A first cooling stage is thermally linked to the radiation shield 16, and provides cooling to a first temperature. In a cryostat with a single shield, the first stage typically cools the shield to about 50K. In cryostats with two shields, they are typically cooled to temperatures of about 80K and 20K. A second cooling stage provides cooling of the cryogen gas to a much lower temperature, typically in the region of 4-10K.
A negative electrical connection 21a is usually provided to the magnet 10 through the body of the cryostat. A positive electrical connection 21 is usually provided by a conductor passing through the vent tube 20.
For fixed current lead (FCL) designs, a separate vent path (auxiliary vent) (not shown in FIG. 1) is provided as a fail-safe vent in case of blockage of the vent tube.
The present invention addresses the consumption of cryogen during transportation of the cryostat, or at any time that the refrigerator 17 is inoperative. When the refrigerator 17 is inoperative, heat from the OVC 14, which is at approximately ambient temperature (250-315K), will flow towards the cryogen vessel 12 by any available mechanism. This may be by conduction through support structures (not illustrated) which retain the cryogen vessel and the radiation shield 16 in position within the OVC; by convection using of gases, typically hydrogen, which may be present in the volume between the cryogen vessel 12 and the OVC 14; or by radiation from the inner surface of the OVC. Much attention is typically paid to reducing all of these possible mechanisms for thermal influx. Support structures are made as long and thin as mechanically practicable, and are constructed from materials of low thermal conductivity, to reduce thermal influx by conduction. Care is taken to remove as much gas as possible from the volume between the cryogen vessel and the OVC, although many gases will freeze as a frost on the surface of a cryogen vessel if a very cold cryogen such as helium is in use. There are some measurements of hydrogen gas being released from the steel which makes up the cryostat. Any such released hydrogen will degrade the OVC vacuum. However, in the case of a helium cryogen, the hydrogen is solid during normal operation and will only be released as a gas if the cryogen vessel heats up to above about 20K.
One or more thermal radiation shields 16 are provided to intercept thermal radiation from the OVC. Any resultant heating of the thermal radiation shield is removed by the refrigerator 17. Further thermal insulation may be provided, such as the well-known “super-insulation”: multilayered insulation of aluminized polyester sheet, typically aluminized polyethylene terephthalate sheet, placed in a layer between the cryogen vessel and the thermal shield 16; or between the thermal shield 16 and the OVC; or both.
In operation, cryogen liquid in cryogen vessel 12 boils, keeping the cooled equipment 10 at a constant temperature, being the boiling point of the cryogen. Refrigerator 17 removes heat from the cryogen gas and the thermal shield 16. Provided that the cooling power of the refrigerator is sufficient to remove any heat generated by the cooled equipment and any heat influx reaching the cryogen vessel, the cooled equipment 10 will remain at its steady temperature, and cryogen will not be consumed.
A difficulty arises during transportation of the cryostat, when the refrigerator is switched off; or at any other time that the refrigerator 17 is inoperative. With the refrigerator inoperative, any heat influx reaching the cryogen vessel, and any heat generated within the cryogen vessel, will cause cryogen liquid to boil. As the refrigerator is inoperative, the boiled-off cryogen cannot be re-condensed into liquid, and will vent to atmosphere through vent tube 20 or the auxiliary vent. In the case of superconducting magnets, for example as used in Magnetic Resonance Imaging (MRI) systems, liquid helium is typically used as the cryogen. Liquid helium is expensive, and difficult to obtain in some parts of the world. It is also a finite resource. For these reasons, it is desired to reduce the consumption of helium cryogen during transport or at other times that the refrigerator 17 is not operating.
It is of course possible to transport the cryostat and the equipment 10 at ambient temperature, empty of cryogen. This will avoid the problem of cryogen consumption during transport. However, the equipment 10 and indeed the cryostat itself will need to be cooled on arrival at its destination. Such cooling is a skilled process, and on-site cooling has been found to be very expensive. Furthermore, the quantity of cryogen required to cool the equipment and cryostat from ambient temperature on arrival at an installation site has been found to far exceed current consumption rates over a reasonable transport time. Typical current systems are able to travel for at least 30 days without the refrigerator operating, and without the liquid cryogen boiling dry.
Current known solutions consume approximately 2.5-3.0% of cryogen inventory per day of transit time. On current systems, this may equate to a consumption of 50 liters of liquid helium per day. The present invention aims to reduce this level of consumption, and so increase the maximum transport time, simplifying the logistics of delivering a cooled equipment to a destination and/or reducing the consumption of cryogen.
Known attempts to address this problem have met with difficulties. Some of the known attempts to address this problem will be briefly discussed.
A second thermal radiation shield, concentric with first thermal shield 16 may be provided. This has been found somewhat effective in reducing thermal influx to the cryogen vessel, but has required increased size of OVC, and caused increased manufacturing costs.
A thermally conductive pipe has been run around the thermal shield, carrying escaping cryogen gas. As the gas is at a temperature little above its boiling point, which is about 4K, such arrangements serve to cool the thermal shield. This has been somewhat effective at reducing thermal influx to the cryogen vessel. Such an arrangement is described, for example, in U.S. Pat. No. 7,170,377 and UK patent application GB 2 414 536, but has also required increased size of OVC to accommodate the thickness of the conductive pipe. Increased manufacturing costs also resulted from the additional assembly effort, and the material and labor costs of increasing the size of the OVC.